1. Field of the Invention
The present invention relates to an optical system and an optical apparatus having the same. The present invention is suitable, for example, for a digital still camera, a digital video camera, a surveillance camera, a film-based camera, and a projector.
2. Description of the Related Art
As an image pickup optical system for use with an optical apparatus, such as a digital camera and a video camera, there are known a variety of optical systems each having a back focus equal to or longer than a focal length. For example, a wide variety of image pickup optical systems are known such as a double gauss type, a retro focus type, an ortho-meta type, a plasmat type, and a hexanon type.
Sadahiko Tsuji, “All about Lens Design” Dempa Newspaper Corporation, 2006, pp. 271, 288, and 230 (referred to as “Tsuji” hereinafter) discloses an image pickup optical system having a focal length from 28 mm to 50 mm (and half an image pickup angle of view from 23° to 38°). This image pickup optical system is required to have a compact overall size and a high optical performance over the screen. A gauss type image pickup system is known as a compact image pickup optical system having a standard angle of view that approximately meets these requirements. Japanese Patent Laid-Open No. 6-337348 discloses a gauss type image pickup optical system having a short overall lens length which includes, in order from the object side to the image side, a front unit of a positive refractive power, an aperture diaphragm, and a rear unit of a positive refractive power.
The gauss type image pickup optical system usually has an approximately symmetrical lens structure as a whole in which a lens unit of a positive refractive power is arranged on a front (object) side, and a lens unit of a positive refractive power is arranged on a back (image) side. In an attempt to obtain a wider angle of view with this image pickup optical system, it is difficult to maintain long the back focus. On the other hand, in an attempt to maintain long the back focus, a variety of aberrations, in particular a curvature of field, accordingly increases and their corrections become difficult. In addition, the entire optical system comes to have a larger size.
As well as these image pickup optical systems, Tsuji also discloses a plasmat type image pickup optical system including in total six lens units in which the object side is replaced with the image side. This image pickup optical system has an approximately symmetrical lens structure in which a lens unit of a positive refractive power is arranged on the front side, and a lens unit of a positive refractive power is arranged on the rear side, similar to the above gauss type image pickup optical system. Thus, similar problems occur in an attempt to obtain a wider angle of view, as described above.
It is difficult for the image pickup optical systems disclosed by Tsuji to realize both a wider angle of view with a shorter overall lens length because the focal length has an approximately inverse relationship with the overall lens length. In addition, in an attempt to obtain a wider angle of view, it is difficult to maintain long the back focus. Moreover, the image pickup optical systems disclosed by Tsuji correct an aberration or the curvature of field over the entire screen using a swelling spherical aberration curve. According to the corrected image pickup optical system, a focus moving amount increases as an aperture value (F value) increases relative to the focus position at the maximum aperture state.
When the correction is made using the swelling spherical aberration curve, it becomes difficult to form a uniform image plane at peripheral angles of field in the screen. In general, the gauss type image pickup optical system can achieve a compact overall size and a wider angle of view to some extent. However, as a much wider angle of view is sought, a spherical aberration and a curvature of field increase, and it becomes difficult to obtain a high optical performance over the entire screen.
In order to realize a compact size and a wide image pickup angle of view in the gauss type and modified gauss type image pickup optical systems, the well-balanced corrections of the spherical aberration and the curvature of field are important. In order for such an image pickup optical system to obtain a high optical performance over the entire screen, it is important to properly set the lens structure of the approximately symmetrical lens units on the object and image sides of the aperture diaphragm. In particular, the lens structure of the lens unit located on the object side of the aperture diaphragm is important. If this lens structure is improper, it becomes difficult to properly correct the spherical aberration and the curvature of field and to obtain a high optical performance.